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<td nowrap="true" class="pageTitle">指南：用例模型中的活动图</td><td width="100%">
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<td valign="top">活动图可表示用例中的流程。该指南描述了如何在此情况下使用活动图。</td>
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<div class="sectionHeading">关系</div>
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<th scope="row" class="sectionTableHeading">相关元素</th><td class="sectionTableCell">
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<a href="./../../../core.base_rup/tasks/detail_a_use_case_D9BECBF4.html" guid="{536BC669-A8FB-426D-88FF-CBEA806A9066}">详细描述用例</a>
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<a href="./../../../core.base_rup/workproducts/rup_usecase_A5D30E62.html" guid="{B1526BC5-E346-42CB-A08A-3C0D7F382407}">用例</a>
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<div class="sectionHeading">主要描述</div>
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<p>
    用例的事件流描述为了向参与者提供价值，系统需要完成的工作。它由一系列任务组成，这些任务一起为参与者产生某些结果。事件流由基本流以及一个或若干备选流组成。
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<p>
    使用活动图有助于以图形化的方式描述用例的事件流。这样的图显示：
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    <li>
        <b>活动状态</b>，代表事件流中的任务或步骤的执行情况。
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    <li>
        <a id="XE_transition_(in_activity_diagrams)__in_business_use-case_model" name="XE_transition_(in_activity_diagrams)__in_business_use-case_model" class="index" key="transition (in activity diagrams)" text="in business use-case model"></a><b>转移</b>，显示活动状态的先后顺序。此类转移有时称为完全转移，因为它与转移不同：它不要求显式触发事件，它由活动状态表示的任务完成来触发。
    </li>
    <li>
        <a id="XE_decision_(in_activity_diagrams)__business_use-case_model" name="XE_decision_(in_activity_diagrams)__business_use-case_model" class="index" key="decision (in activity diagrams)" text="business use-case model"></a><b>决策</b>，为其定义了一组<b>警戒条件</b>。这些警戒条件控制一旦任务完成后所遵循的（一组备选转移中的）某一转移。决策和警戒条件使您能够在用例的事件流中显示
        <b>可选线程</b>。
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    <li>
        <b>同步条</b>，可以使用它们显示并行子流。同步条使您能够在用例的事件流中显示<b>并发线程</b>。
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    <img height="495" alt="附带文本中描述的图。" src="./../../../core.base_rup/guidances/guidelines/resources/md_act.gif" width="510" />
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<p class="picturetext">
    自动柜员机（ATM）的用例模型中的用例“回撤资金”的简化活动图。
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<p>
    活动图是状态表图的特殊情况，其所有或大部分状态都是活动状态，其中所有或大部分的转移由源状态中的操作完成而触发。<br />
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